Process for the surface treatment of workpieces of magnesium and magnesium base alloys



Batented Jan. 21, 1941 eAr-ENT 'l orrlca PROCE S S FOR THE v 12mmMAGNESIUM BASE ALLOYS Adolf Beck, GfistavSiebel, and Eduard Nalitiiiil,Bitter-fold, Germany, assignors, by 'mesne a signments, to MagnesiumDevelopment Corporation, a corporation of Delaware No Drawing. motionSeptember 1,1939, Serial No. 223,160. In Germany September 2,

9 Claims. (01. 148-6) This invention relates to a process for thesuriacetreatxfient of workpieces of magnesium and magnesium base alloysfor thepurpose of forming a protective coating thereon.

Known processes for building up corrosionresisting coatings on magnesiumand magnesium base alloys, consist in treating the metal to be protectedwith solutions of inorganic compounds, particularly chromates, fluorides'or phosphates. The resulting coatings are generally very. thin andeasily damaged; it it be possible at all to produce thicker layers,these are porous and brittle and therefore readily flake off from themetal.

A long time ago it was pointed out that coatings oi. magnesium oleate orresinate produced on magnesium or magnesium base alloys possessed anextremely high resistance tofchemical and mechanical influences, andcould 'walsO be coloured and polished. As regards the produc tion ofsuch coatings, however, there was merely an indication that this couldbe done by heating the parts to be coated for a prolonged period in anoil-water-soap emulsion. 7

According to the present invention corrosion resisting layers areproduced on magnesium and magnesium base alloys by treating the metalwith aqueous solutions of those salts of carboxylic acid or phenolateswhich react with magnesium to form substantially insoluble compoundsattemperatures in the vicinity of the boilingpoint of the solutions, butpreferably at higher temperatures and in such case, under pressure.

The alkali metal salts, including the ammonium salts, of the saturatedand unsaturated fatty; acids (including the iso-fatty acids) and oxyfatty acids. containing eight or more carbon atoms, and also. alkalimetal salts, including the ammonium salts, of aromatic carboxylic acidshaving two or more carbon rings, andthe substitution products thereof,are particularly suitable for use in carrying out the present invention.The use of the free fatty and oxy-fattyiacidsl also of the aromaticcarboxylic acids themselves usually fails either because such acids formread: ily soluble magnesium salts or because they are not water-soluble.

The solutions employed in carrying out the invention may, if desired,also contain other suitable solvents, such as ethanol, in addition towater.

It has beenzrimmd-athat the presence in the solutions, of water-solublesulphates or nitrates accelerates the building up of the protectivecoatin'gs, while the presence of water-soluble silicates or ofwater-soluble salts of chromium and manganese, renders the coatingsharder and/or more resistant to corrosion. In a similar manner, thenature of the coatings can be graduated by varying the temperature andpressure.

'The coatings produced in accordance with the present invention can becoloured by adding colouring substances to the solution employed for thetreatment; the colouring substances may,

however, be produced directly in the coating zfoi'med, by employing forthe production of the latter a substance suitable in accordance with theinvention, which substance also acts as a dye component and subsequentlyallowing the solution of a suitable coupling component to act on thecoating.

The hereindescribed process may also be applied to articles which havepreviously been subjected to a surface treatment, for example by adipping process.

When treating magnesium base alloys the thickness and quality of thecoatings depend to a. certain extent, on the proportion of the alloyingconstituents contained in the magnesium, in the sense that the thicknessof .the coating which can be built up varies inversely with the contentof alloying constituents However, the quality of the layer is largelyindependent of the concentration of the solutions employed.

The following examples illustrate the invention:

1; A sheetnof anralloy' containing 2% of manganese-,-remairidermagnesium, is heated in an autoclave' ina.3% aqueous solution ofpotassium Palm-intestin 15 minutes at a temperature of about l'fQf. C. Auniformly yellow coatingabout @102 mm; in thickness and'possessing greatresist'an'ce'to abrasion'fis formed on the sheet, and imparts to-tl'ie'latter good resistance to corrosion, including corrosion by sea water.

The addition of 1% of sodium sulphate to the foregoing solution enablesthe duration of the treatment required to produce a coating oi. the"same thickness to be reduced to 5 minutes.

Similar coatings are obtained in the same manner by using aqueoussolutions oi the alkali metal or ammonium salts of stearic acid, oleicacid or linoleic acid.

2. A sheet of an alloy containing 2% of manganese and 0.4% of cerium,remainder'rnagnesium, is treated in an autoclave with an aqueoussolution oi! sulphoricinoleate (Turkey-red oil) for at least 20 minutesat a temperature of 180 C. The appearance and properties oi the coatingproduced are similar to those oi thecoating produced in accordance withExample 1.

3. A sheet of an alloy containing 6% of aluminium, 1% of zinc, remaindermagnesium is heated at 180 C. for 15 minutes in an autoclave in a 5%aqueous solution or potassium abietate (produced by dissolving pineresin or colophony in hot dilute caustic potash solution). The resultingcoating is brown .and adheres very securely to the metal. In thepolished state it possesses a high gloss and can, if required, be coatedwith a firmly adherent layer of synthetic resin lacquer for the purposeof further increasing the resistance to corrosion.

4. A casting of an alloy containing 4%, of aluminium, 3% of zinc,remainder magnesium, is treated for two hours under normaLpressure atboiling temperature with a 10% aqueous solution of potassium naphthenate(produced by saponifi- 6. A sheet of an alloy containing 6% ofaluminium, 1% of zinc, remainder magnesium, is treated under theconditions described in Example 1, with a 6% aqueous solution of thepotassium salt of p-oxynaphthoic acid anilide. The resulting layer,yellow to green in colour, assumed a bright red colour of great fastnesswhen dipped in a 1% aqueous solution of KB fast red salt.

The chemical resistance of the layer can be increased by using only a 5%solution of the potassium salt of p-oxynaphthoic acid anilide, insteadof a 6% solution, which also contains 2% of sodium palmitate.

7. A sheet of an alloy containing 6% of aluminium, 1% of zinc, remaindermagnesium, is heated in a solution of 40 grams of o-oxyquinoline and 11grams of caustic soda solution in 1 litre of water, for about 30 minutesat a temperature of 160 C. in an autoclave. A hard, firmly adherentyellow coating, which can be polished and which imparts good protectionagainst corrosion, is produced on the metal, and can, if desired, besubsequently compacted in known manner.

What we claim is:

1. A process for theproduction of protective 'coafings upon workpieces.of magnesium and magnesium base alloys, which comprises treating theworkpiece with an aqueous solution of a salt of an aliphatic carboxylicacid containing at least eight carbon atoms, at temperatures at least sohigh as to be in the vicinity of the boiling point of said solution.

2. A process for the production of protective coatings upon workpiecesof magnesium and magnesium base alloys',,which comprises treating theworkpiece with an 'aqueous solution of an ammonium salt of an aliphaticcarboxylic acid containing at least eight carbon atoms, at temperaturesat least so high as to be in the vicinity of the boiling point of saidsolution.

3. A process for the production of protective coatings upon workpiecesof magnesium and magnesium base alloys, which comprises treating theworkpiece with an aqueous solution of an alkali metal salt of anaromatic carboxylic acid containing at least two carbon rings, attemperatures at least so high as to be in the vicinity of the boilingpoint of said solution.

4. A process for the production or protective coatings upon workpiecesof magnesium and magnesium base alloys, which comprises treating theworkpiece with an aqueous solution of an ammonium salt of an aromaticcarboxylic acid containing at least two carbon rings, at temperatures atleast so high as to be in the vicinity of the boiling point of saidsolution.

5. A process for the production of protective coatings upon workpiecesof magnesiumand magnesium base alloys, which comprises treating theworkpiece with an aqueous solution of an alkali metal salt of anaromatic carboxylic acid containing at least two carbon rings andadapted to act as a dye-stuff component, at temperatures at least sohigh as to be in the vicinity of the boiling point of said solution, andthereaftertreating the workpiece with a solution of a component adaptedto produce a dye-stufi.

6. A process for the production of protective coatings upon workpiecesof magnesium and magnesium base alloys, which comprises treating theworkpiece with an aqueous solution of at least one of those easilywater-soluble substances of the group consisting of the salts ofaliphatic and aromatic carboxylic acids and of the phenolates whichreact with magnesium to form substantially water-insoluble compounds, attemperatures at least so high as to be in the vicinity of the boilingpoint oi. said solution.

' 7. A process for the production or protective coatings upon workpiecesof magnesium and magnesium base alloys, which comprises treating theworkpiece with an aqueous solution 01. at least.

one of those easily water-soluble substances of the group consisting ofthe salts of aliphatic and aromatic carboxylic acids andof thephenolates which react with magnesium to form substantiallywater-insoluble compounds, at temperatures at least so high as to be inthe vicinity of the boiling point of the solution, said solution alsocontaining at least one water-soluble salt of the group consisting ofsulphates, nitrates, and silicates.

8. A process for the production of protective coatings upon workpiecesof magnesium and magnesium base alloys, which comprises treating theworkpiece with an aqueous solution of at least one of those easilywater-soluble substances of the group consisting of the salts ofaliphatic and aromatic carboxylic acids and of the phenolates whichreact with magnesium to form substantially water-insoluble compounds, attemperatures at least so high as to be in the vicinity of the boilingpoint of the solution, said solution also containing a water-solublechromium salt.

9. A process for the production of protective coatings upon workpiecesof magnesium and magnesium base alloys, which comprises treating theworkpiece with an aqueous solution of at least one of those easilywater-soluble substances of vicinity of the boiling point of thesolution, said the group consisting of the salts of aliphatic solutionalso containing a water-soluble manand aromatic carboxylic acids and ofthe ganese salt. phenolates which react with magnesium to form 1 AIQOLFBECK.

5 substantially Water-insoluble compounds, at GUSTAV SIEBEL. 5

temperatures at least so high as to be in the EDfiARD NACHIIGALL.

